This application claims the benefits of Japanese Patent Applications P2003-28600 filed on Feb. 5, 2003, and P2002-68862 filed on Mar. 13, 2002, the entireties of which are incorporated by reference.
1. Field of the Invention
The present invention relates to a vibrator and vibratory gyroscope.
2. Related Art Statement
It has been studied to use a vibratory gyroscope as a turning angular rate sensor employed in a vehicle control system of an automobile body based on a vehicle turning rate feedback system. Such a system detects the direction of a steering wheel itself by a turning angle of the steering wheel. Simultaneously, the turning rate of the car vehicle is detected by the vibratory gyroscope. The system finds a difference by comparing the direction of the steering wheel with the actual body turning velocity, and attains a stable vehicle control by correcting a wheel torque and a steering angle on the basis of this difference.
In a vehicle control system, a vibratory gyroscope and its vibrator is subjected to a wide temperature range including high and low temperatures, for example from minus 40xc2x0 C. to plus 85xc2x0 C. Even after the resonance frequency of a pair of bending vibration pieces is adjusted at a specific value, the resonance frequency may be substantially changed or deviated when the temperature is changed to a higher or lower temperature. As a result, so-called zero point temperature drift may be induced.
The assignee filed a Japanese patent publication 2001-12952A. In the publication, a tapered part is provided on the base part of each of the sides of a bending vibration piece for preventing the zero point temperature drift.
The inventors have studied such technique and found the following problems, depending on the material of the vibrator. That is, as described in Japanese patent publication 2001-12952A, tapered parts are provided in the bases of the side faces of a bending vibration piece so that the tapered parts have substantially the same planar shape. It is considered that the vibration mode of the bending vibration piece may be made more symmetrical to reduce the temperature drift. When the zero point temperature drift is measured for each of vibrators actually fabricated, however, the drift may be deviated among the vibrators. The deviation of zero point temperature drift may be increased so that the yield of off-specification products may be increased.
The reasons are as follows. Even when the zero point temperature drift of the vibrator cannot be made zero, it is possible to substantially cancel the drift by providing a correction circuit of the drift in a detection circuit of a vibratory gyroscope when the drift is made substantially constant. Even if the deviation of the drift of the vibrator actually fabricated is larger, the drift may be cancelled by means of the correction circuit in one vibrator. In this case, however, the drift in another vibrator may be considerably different from that in the one vibrator in which the drift is cancelled with the circuit. It is not possible to cancel the drift in another vibrator so that the performance of a vibratory gyroscope having this vibrator may be made out of a specification.
An object of the present invention is to reduce the zero point temperature drift of a vibrator and the deviation or change of the drift among vibrators fabricated.
A first invention provides a vibrator having a driving vibration piece, a detection vibration piece and a base portion connecting the driving and detection vibration pieces. The driving vibration piece is composed of a bending vibration piece having a pair of surfaces parallel with each other and a pair of side faces. The driving vibration piece has a ratio (WD/TD) of a width WD to a thickness TD of not smaller than 1.5 and not larger than 2.4.
A second invention provides a vibrator having a driving vibration piece, a detection vibration piece and a base portion connecting the driving and detection vibration pieces. The driving vibration piece is composed of a bending vibration piece having a pair of surfaces substantially parallel with each other and a pair of side faces. The driving vibration piece has a ratio (WD/TD) of a width WD to a thickness TD of not smaller than 0.3 and not larger than 0.8.
The invention further provides a vibratory gyroscope having any one of the vibrators.
The inventors have studied the cause of the deviation of the zero point temperature drift among vibrators and the increase of the drift. They finally found that it is possible to reduce the drift and the deviation or change of the drift among vibrators by adjusting a ratio (WD/TD) of a width WD to a thickness TD of a driving vibration piece at a value from 1.5 to 2.4. The invention is based on the discovery.
Although the reason is not clear, it may be considered as follows. That is, a wafer made of, for example, a piezoelectric single crystal, is etched for forming the outer shape of a vibrating piece. For example in the etching step, a photoresist is applied on both of upper and lower faces of the wafer. Photo masks are mounted on the photoresists, and the photo masks on the upper and lower faces are aligned with each other. The photoresist are exposed to light to harden it, and the photo masks are removed to complete the patterning of the photo resists. The wafer is then etched so that an outline corresponding with the pattern of the photoresist is formed on the wafer.
When the photo masks are mounted on both of the upper and lower faces of the wafer and aligned with each other, alignment error may occur between the photo masks. In this case, as shown in FIG. 1, the cross sectional shape of the vibrating piece 2 is deformed to a shape of a parallelogram (see dotted lines). When the vibrating piece 2 is driven in a direction shown in an arrow A, unnecessary vibration component in the direction of z-axis is induced so that torsion vibration is induced in the detection vibration piece. Such noise component may be substantially changed depending on temperature to generate the zero point temperature drift. The deformation of the cross sectional shape of the vibration piece 2 is considered to be a cause of the drift.
According to the discovery, it is possible to reduce the drift and the deviation of the drift among vibrators by adjusting the ratio WD/TD of the width WD to the thickness TD of a driving vibration piece 2A at a value not smaller than 1.5, as shown in FIG. 2. Such effect may be due to the reduction of the unnecessary vibration component in the direction of z-axis by adjusting WD/TD at a value not smaller than 1.5, that is, by applying a wider cross sectional shape.
Further, when WD/TD exceeds 2.4, (when the width is too large in other words), torsion vibration component may be induced so that the unnecessary vibration component in Z-axis made considerable to increase the drift. It is thus necessary that WD/TD be not larger than 2.4.
Further, according to the discovery, it is possible to reduce the drift and the deviation of the drift among vibrators by adjusting the ratio WD/TD of the width WD to the thickness TD of a driving vibration piece 2A at a value not larger than 0.8, as shown in FIG. 4. Such effect may be due to the reduction of the unnecessary vibration component in the direction of z-axis by adjusting WD/TD at a value not larger than 0.8, that is, by applying a narrower cross sectional shape.
Further, when WD/TD is below 0.3 (when the width of the cross sectional shape is too narrow in other words), torsion vibration component may be induced so that the unnecessary vibration component in Z-azis made considerable to increase the drift. It is thus necessary that WD/TD be not smaller than 0.3.
It is considered that the effects of reducing zero-point temperature drift is considerable as the difference of the thickness and width of the vibration piece is smaller. When the thickness and width of the vibrating piece is substantially identical, the vibration component in the direction of width and that in the direction of thickness have the substantially same characteristic resonance frequency with each other. Mode coupling may be thus easily induced. When the vibrating piece is driven along a specified plane, the mode coupling may generate a large vibration component in the direction of z-axis.
These and other objects, features and advantages of the invention will be appreciated upon reading the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be made by the skilled person in the art.